RAID arrays are something we are hearing more and more every day. Once it was for system administrators, nowadays it is for enthusiasts and possibly it will be for every user. Inside is a detailed look at RAID arrays and RAID levels.
Let’s start our article with the question “what does RAID stand for.” RAID is an abbreviation of Redundant Array of Independent Disks. At first, the idea behind was to achieve high storage capacities by combining disks in an array by using off the shelf (inexpensive) disks. Therefore it was named as Redundant Array of Inexpensive Disks. Later on, it was changed to independent instead of inexpensive to not to let the users think that the system is just a low-cost system.
RAID is actually a term that describes a storage system which data is divided and/or replicated between hard disks. To further explain which system is used, a number is added after the word RAID; such as RAID 0, RAID 1, RAID 5 etc., which we will discuss in the next section.
The first word in RAID, redundancy, is achieved in such a way that either the same data is written to all disks (mirroring) or written in such a way that extra data is calculated and written to allow recovering data in case of a loss of a hard disk (this extra data is called parity). With parity, when a hard disk failure occurs, a disk is replaced and the data is recalculated and written.
The most common RAID arrays are as follows:
- RAID 0: Minimum two disks are required. The data is distributed to two disks, and this takes advantage of higher disk read/write speeds. If one disk fails, there is no chance of recovering data, since some parts are lost and there is no parity. In a RAID 0 array, the capacity of the total array is half of the disk sizes. For example if you use two 1 Gigabyte disks in a RAID 0 array, the total size is not 2 Gigabytes, but 1 Gigabyte.
- RAID 1: Minimum two disks are required. The data is written exactly to two disks, which mean that if one disk fails, you have an exact copy on the other. Again, if you have two 1 Gigabyte disks in a RAID 1 array, the total capacity is not 2 Gigabytes, but 1 Gigabyte.
- RAID 5: Minimum three disks are required. The disks hold both data and parity, so if any disk fails, it can be replaced and the data could be recovered from the data in the other disks. Total storage capacity with three 1 Gigabyte drives is 2 Gigabytes. The system can recover from one hard disk failure.
- RAID 6: Minimum four disks are required. The data and the parity information is distributed to all the disks, and the parity data is distributed twice (dual distribution). RAID 6 arrays can recover if two hard disks fail. If the array is arranged with four 1 Gigabyte disks, the total storage capacity of the array is 2 Gigabytes (4 x 1 Gigabytes, minus 2 x 1 Gigabytes dual parity: (4 x 1) - (2 x 1) = 2 Gigabytes)
There are also other RAID arrays such as RAID 3, RAID 10, RAID 01, RAID 4, RAID 1.5, RAID 5E etc., which have their own arrangements and complexities.
Read on for RAID implementations.